This prom season, high schools are getting serious about prom dress codes.

For example: We’ve been reading about some schools that are putting up posters in the hallways, or distributing catalogs full of photos, showing what’s forbidden at the prom. For girls, the “forbidden” list usually includes dresses with plunging necklines, low-slung backs, thigh-high slits, or any gown that shows skin around the rib cage. For guys, the bans include wearing jeans or pants that sag.

Why the crackdown on prom dress codes? Experts say it comes from the fact that today’s proms tend to be full of girls wearing skintight, or “barely there” outfits! As one student in Georgia put it: “The more you show, the better it is,” which helps explain why a-third of the prom dresses sold by today’s mall retailers are dresses with low backs, or midriff-baring cutouts.

So, to fight back, school officials say they want to make it crystal clear in advance what will and won’t be allowed at the prom. Some schools are promising to have rulers on hand, to help verify that dresses are no shorter than 3 inches above the knees. Others are making students sign a contract, promising they’ll abide by the rules or be sent home! That’s a big deal when you consider the average high-school student will spend about $800 on prom this year - including clothing, transportation, and pictures.

The good news is that the dress code crackdowns are working: For example, the website PromGirl.com is reporting a spike in returned dresses, from girls claiming they broke the rules, and now want to buy a more appropriate dress.